1. Field of the Invention
The present invention relates to a liquid crystal display apparatus driving method capable of preventing degradation in display quality of a liquid crystal display apparatus.
2. Description of the Related Art
A liquid crystal apparatus has power-consumption and portability advantages over other known image display apparatuses, and thus development in the liquid crystal apparatus field has been actively pursued. FIG. 11 is a timing chart of voltage waveforms, illustrating a prior art TFT liquid crystal display apparatus driving method. In the figure, Line L1 represents a waveform of a voltage applied to a pixel electrode; Line L2 represents a waveform of a scanning voltage inputted to a gate electrode; Line L3 represents a waveform of a display voltage inputted to a source electrode; Line L4 represents a reference potential, i.e. an intermediate potential of the display voltage; and Line L5 represents a counter potential of a common electrode.
When a positive gate-on voltage is applied to the gate electrode, the TFT is turned on, and thereby a display voltage is fed from the source electrode so as to be inputted via a drain electrode to the pixel electrode acting as a reflecting electrode. As a result, pixels are turned on. The TFT is kept in the ON state for a predetermined period of time, and, after a display voltage is applied to the pixel electrode, a gate-off voltage is applied to the gate electrode. Hereupon, the power supply to the pixel electrode is completed. The pixel electrode is, by exploiting the holding characteristics of the liquid crystal, maintained in a predetermined-voltage applied state until a gate-on voltage is applied once again to the TFT, i.e. over xe2x80x9cgate-offxe2x80x9d periods. When a gate-off voltage is applied to the gate electrode, due to subsequently-described parasitic capacitance Cgd, the voltage carried by the pixel electrode varies and takes a voltage variation value of xcex94V1 calculated from the following formula:
xcex94V1=xcex94Vgxc3x97{Cgd/(Cgd+Clc+Ccs)}xe2x80x83xe2x80x83(1)
Note that, in the above formula (1), xcex94V1 represents a value of voltage variation resulting from the parasitic capacitance; xcex94Vg represents the displacement amount of the potential of the gate voltage (gate-on voltage relative to gate-off voltage); Cgd represents static capacitance of the parasitic capacitance; Clc represents static capacitance of liquid crystal capacitance; and Ccs represents static capacitance of hoplding capacitance.
Such voltage variation as occurs in the pixel electrode leads to a DC voltage component, and this DC voltage component acts upon a liquid crystal layer. The action of the DC voltage component exerted on the liquid crystal layer causes the liquid crystal to exhibit polarization, which results in degradation in the reliability of the liquid crystal. As a result, the display surface suffers from an image persistence. Hereinafter, a DC voltage component resulting from voltage variation occurring in the pixel electrode is referred to as the first DC voltage component xcex94V1.
To prevent the first DC voltage component xcex94V1 from acting upon the liquid crystal layer, in the prior art, the circuit configuration of the liquid crystal display apparatus is designed such that the first DC voltage component xcex94V1 calculated from the formula (1) is corrected beforehand. In other words, the potential of the common electrode to which a counter electrode is connected standing at the reference potential (i.e. the intermediate potential of the display voltage indicated by the line L4) level is shifted by an amount of the first DC voltage component xcex94V1 in a negative potential direction so as to be initially set at the counter potential level indicated by Line L5.
Voltage variation resulting from the parasitic capacitance Cgd is possibly suppressed by adopting such a power source circuit configuration as shown in FIG. 12. In this case, Hi-voltage and Low-voltage are outputted in response to a control signal Vin at given intervals. When High-voltage is fed, a switch S is turned on, and thereby a voltage of a power source P1 is applied to a capacitor C. After a lapse of a predetermined period of time, Low-voltage is outputted in response to the control signal Vin, and thereby a GND (ground) potential is applied to the capacitor C. By applying to the capacitor C a power source voltage and a GND voltage at predetermined intervals, an alternating voltage is outputted from the capacitor C to the common electrode side (output signal: Vout). Then, a specific voltage is applied to the alternating voltage so that the voltage variation resulting from the parasitic capacitance Cgd of the capacitor C is corrected.
An application voltage refers to a voltage which is outputted from a power source P2 and is then fed toward a resistance R3 side through divided resistance, i.e. resistances R1 and R2. FIG. 13 shows a waveform of the output signal Vout. The waveform of the output signal Vout is formed as a composite waveform created by linking the waveform of the alternating voltage from the capacitor C and the waveform of the DC voltage from the power source P2. By applying a correction voltage to the common-electrode side in that way, the influence of the voltage variation resulting from the parasitic capacitance Cgd can be suppressed.
However, application of a correction voltage requires an additional power source, like the power source P2 shown in FIG. 12. In addition, a negative power source is required for correcting the alternating voltage of the common electrode. This leads to an undesirable increase of power consumption.
A DC voltage component acting upon the liquid crystal layer is caused not only by the above-described parasitic capacitance Cgd but also by asymmetricity in characteristics between an active matrix substrate and a counter substrate that have sandwiched therebetween the liquid crystal layer. A DC voltage component resulting from the asymmetricity between the active matrix substrate and the counter substrate acts upon the liquid crystal layer constantly. Hereinafter, a DC voltage component resulting from the difference in characteristics between the mutually-opposing substrates is referred to as the second DC voltage component xcex94V2.
The asymmetricity in characteristics between the substrates includes: the difference in thickness between the active-matrix-substrate-side alignment film and the counter-substrate-side alignment film; the difference in material between the active-matrix-substrate-side alignment film and the counter-substrate-side alignment film (observed in the case of hybrid orientation); and the difference in material between two electrodes opposed to each other with a liquid crystal layer therebetween, like an Al-made active-matrix-substrate-side reflecting electrode and an ITO-made counter-substrate-side transparent electrode in a reflection-type liquid crystal display apparatus. Of these factors, in particular, the asymmetricity defined by the difference in material between electrodes opposed to each other with a liquid crystal layer therebetween causes the largest second DC voltage component xcex94V2.
Moreover, the second DC voltage component xcex94V2 resulting from the difference in material between the electrodes cannot be obtained by calculation. Therefore, it takes much time to adjust the potential of the common electrode properly, and, during the adjustment, the second DC voltage component xcex94V2 continues to act upon the liquid crystal layer. This leads to degradation in the reliability of the liquid crystal display apparatus and causes problems such as occurrence of an image persistence.
Further, Japanese Unexamined Patent Publication JP-A 2-64525 (1990) discloses a technique for preventing occurrence of the second DC voltage component xcex94V2 by making the active-matrix-substrate-side alignment film identical in material and thickness with the counter-substrate-side alignment film. However, the prior art technique disclosed in this publication failed to come up with satisfactory solutions to the above-described problem particularly encountered by a liquid crystal display apparatus which necessitates electrodes made of different materials, like a reflection-type liquid crystal display apparatus. Moreover, the publication makes no reference to a technique for solving the above-described problem and improving display quality for a case where an active matrix substrate differs in characteristics from a counter substrate.
Accordingly, an object of the invention is to provide a liquid crystal display apparatus driving method capable of preventing degradation in display quality due to occurrence of a DC voltage component.
The invention provides a method for driving a display apparatus, the display apparatus comprising:
a first substrate having a first electrode;
a second substrate having a second electrode, the second electrode being opposed to the first electrode; and
a display medium layer whose display condition is changed in accordance with a voltage component applied between the first electrode and the second electrode,
wherein a correction voltage is applied beforehand so as to correct the voltage component resulting from difference in characteristics between the first and second substrates.
According to the invention, a display apparatus comprises: a first and a second substrate mutually opposed; a first electrode provided in the first substrate; a second electrode provided in the second substrate; and a display medium layer interposed between the first and second substrates, wherein a correction voltage is applied beforehand so as to correct a voltage component, resulting from the difference in characteristics between the first and second substrates, which acts upon the display medium layer. With this construction, a voltage component resulting from the difference in characteristics between the substrates is cancelled out, thereby protecting the display medium layer against the voltage component. As a result, occurrence of troubles such as an image persistence is successfully prevented, so that the reliability of the display apparatus improves.
The invention further provides a display apparatus comprising:
a first substrate having a first electrode;
a second substrate having a second electrode, the second electrode being opposed to the first electrode; and
a display medium layer whose display condition is changed in accordance with a voltage component applied between the first electrode and the second electrode,
wherein a correction voltage is applied beforehand so as to correct the voltage component resulting from difference in characteristics between the first and second substrates.
According to the invention, a display apparatus comprises: a first and a second substrate mutually opposed; a first electrode provided in the first substrate; a second electrode provided in the second substrate; and a display medium layer interposed between the first and second substrates, wherein a correction voltage is applied before hand so as to correct a voltage component, resulting from the difference in characteristics between the first and second substrates, which acts upon the display medium layer. With this construction, a voltage component resulting from difference in characteristics between the substrates is cancelled out, thereby protecting the display medium layer against the voltage component. As a result, occurrence of troubles such as an image persistence is successfully prevented, so that the reliability of the display apparatus improves.
According to the invention, a voltage component resulting from the difference in characteristics between the substrates is corrected beforehand, and therefore the display medium layer is protected against the voltage component. As a result, occurrence of troubles such as an image persistence is successfully prevented, so that the reliability of the display apparatus improves.
In the invention, it is preferable that the first electrode is formed as a pixel electrode, and supply/cutoff of display voltages to the pixel electrode is controlled by a thin-film transistor, that the second electrode is formed as a counter electrode to which a common electrode is connected, and that a potential of the common electrode standing at a reference potential (i.e. an intermediate potential of the display voltages) level is shifted by an amount of a first DC voltage component xcex94V1 resulting from voltage variation caused by a parasitic capacitance of the thin-film transistor so as to be set at a counter potential level, and the potential set at the counter potential is further shifted by an amount of a second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates so as to be initially set at a correction potential level.
According to the invention, the potential of the common electrode is shifted by an amount of the first DC voltage component xcex94V1 resulting from the parasitic capacitance of the thin-film transistor so as to be set at the counter potential level, and the potential set at the counter potential level is further shifted by an amount of the second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates so as to be initially set at the correction potential level. This makes it possible to cancel out the second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates (such as the difference in material and film thickness between their electrodes or alignment films) as well as the first DC voltage component xcex94V1 resulting from voltage variation caused by parasitic capacitance. As a result, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, and thus occurrence of troubles such as an image persistence is substantially prevented, so that the reliability of the liquid crystal display apparatus improves. Moreover, no additional power source is required and accordingly reduction in power consumption is achieved.
Moreover, according to the invention, it is possible to correct beforehand the second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates as well as the first DC voltage component xcex94V1 resulting from voltage variation caused by parasitic capacitance. Therefore, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, and thus occurrence of troubles such as an image persistence is substantially prevented. As a result, the display quality and reliability of the liquid crystal display apparatus improve.
In the invention, it is preferable that a work function of the first electrode is set to be smaller than a work function of the second electrode.
According to the invention, since the work function of the first electrode is set to be smaller than that of the second electrode, a DC voltage component ascribable to the work functions of the electrode materials is minimized.
Moreover, according to the invention, a DC voltage component ascribable to the work functions of the electrode materials is minimized.
The invention still further provides a method for driving a liquid crystal display apparatus, the liquid crystal display apparatus comprising:
a first substrate having a first electrode;
a second substrate having a second electrode, the second electrode being opposed to the first electrode; and
a liquid crystal layer interposed between the first substrate and the second substrate,
wherein a correction voltage is applied beforehand so as to correct a DC voltage component, resulting from difference in characteristics between the first substrate and the second substrate, which acts upon the liquid crystal layer.
According to the invention, a liquid crystal display apparatus comprises: a first and a second substrate mutually opposed; a first electrode provided in the first substrate; a second electrode provided in the second substrate; and a liquid crystal layer interposed between the first and second substrates, wherein a correction voltage is applied beforehand so as to correct a DC voltage component, resulting from the difference in characteristics between the first and second substrates, which acts upon the liquid crystal layer. With this construction, a DC voltage component resulting from the difference in characteristics between the substrates is cancelled out, and thus the liquid crystal layer is protected against the DC voltage component. As a result, occurrence of troubles such as an image persistence is successfully prevented, so that the reliability of the liquid crystal display apparatus improves.
In the invention, it is preferable that the difference in characteristics between the substrates includes the difference in material between the pixel electrode and the counter electrode.
According to the invention, even in a liquid crystal display apparatus in which the first-substrate-side pixel electrode and the second-substrate-side counter electrode are made of different materials, like a reflection-type liquid crystal display apparatus, a DC voltage component is successfully cancelled out, so that the display quality improves.
Moreover, according to the invention, even in a liquid crystal display apparatus in which the first-substrate-side pixel electrode and the second-substrate-side counter electrode are made of different materials, like a reflection-type liquid crystal display apparatus, a DC voltage component is prevented from acting upon the liquid crystal layer, so that the display quality improves.
In the invention, it is preferable that the difference in characteristics between the substrates includes the difference in film thickness between the pixel electrode and the counter electrode.
According to the invention, even in a case where the pixel electrode differs in film thickness from the counter electrode, a DC voltage component is successfully cancelled out, so that the display quality improves.
Moreover, according to the invention, even in a case where the pixel electrode differs in film thickness from the counter electrode, a DC voltage component is prevented from acting upon the liquid crystal layer, so that the display quality improves.
In the invention, it is preferable that the first substrate has a first alignment film and the second substrate has a second alignment film, and that the difference in characteristics between the substrates includes the difference in material between the first alignment film and the second alignment film.
According to the invention, even in a case where the first-substrate-side first alignment film and the second-substrate-side second alignment film are made of different materials, a DC voltage component is successfully cancelled out, so that the display quality improves.
Moreover, according to the invention, even in a case where the first-substrate-side first alignment film and the second-substrate-side second alignment film are made of different materials, a DC voltage component is prevented from acting upon the liquid crystal layer, so that the display quality improves.
In the invention, it is preferable that the first substrate has a first alignment film and the second substrate has a second alignment film, and that the difference in characteristics between the substrates includes the difference in film thickness between the first alignment film and the second alignment film.
According to the invention, even in a case where the first-substrate-side first alignment film differs in thickness from the second-substrate-side second alignment film, a DC voltage component is successfully cancelled out, so that the display quality improves.
Moreover, according to the invention, even in a case where the first-substrate-side first alignment film differs in thickness from the second-substrate-side second alignment film, a DC voltage component is prevented from acting upon the liquid crystal layer, so that the display quality improves.
In the invention, it is preferable that the first electrode is formed as a pixel electrode and supply/cutoff of display voltages to the pixel electrode is controlled by a thin-film transistor; that the second electrode is formed as a counter electrode to which a common electrode is connected; and that a potential of the common electrode standing at a reference potential (i.e. an intermediate potential of the display voltages) level is shifted by an amount of a first DC voltage component xcex94V1 resulting from voltage variation caused by the parasitic capacitance so as to be set at a counter potential level, and the potential set at the counter potential level is further shifted by an amount of a second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates so as to be initially set at a correction potential level.
According to the invention, the potential of the common electrode is shifted by an amount of the first DC voltage component xcex94V1 resulting from the parasitic capacitance of the thin-film transistor so as to be set at the counter potential level, and the potential set at the counter potential is further shifted by an amount of the second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates so as to be initially set at the correction potential level. This makes it possible to cancel out the second DC voltage component xcex94V2 resulting from the difference in characteristics between the substrates (such as the difference in material and thickness between their electrodes or alignment films), as well as the first DC voltage component xcex94V1 resulting from voltage variation caused by the parasitic capacitance. Therefore, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, and thus occurrence of troubles such as an image persistence is satisfactorily prevented, so that the display quality and reliability of the liquid crystal display apparatus improve.
In the invention, it is preferable that a work function of the first electrode is set to be smaller than a work function of the second electrode.
According to the invention, since the work function of the first electrode is made smaller than that of the second electrode, DC voltage components ascribable to the work functions of both electrodes are kept small.
In the invention, it is preferable that, in a case where the pixel electrode is a reflecting electrode and the counter electrode is a transparent electrode, the potential of the common electrode standing at the counter potential level is shifted by an amount of the second DC voltage component xcex94V2 in a positive potential direction so as to be initially set at the correction potential level.
According to the invention, in a case where a reflecting electrode is used as the pixel electrode and a transparent electrode is used as the counter electrode, a positive second DC voltage component xcex94V2 is generated in the liquid crystal layer. To cancel this out, the potential of the common electrode standing at the counter potential level is shifted in a positive potential direction to the correction potential level. In this way, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, so that the display quality improves.
Moreover, according to the invention, in a case where a reflecting electrode is used as the pixel electrode, a positive second DC voltage component xcex94V2 is generated. Thus, the potential of the common electrode standing at the counter potential level is shifted in a positive potential direction to the correction potential level. In this way, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, so that the display quality improves.
In the invention, it is preferable that, in a case where the pixel electrode is a transparent electrode and the counter electrode is a reflecting electrode, the potential of the common electrode standing at the counter potential level is shifted by an amount of the second DC voltage component xcex94V2 in a negative potential direction so as to be initially set at the correction potential level.
According to the invention, in a case where a transparent electrode is used for each of the pixel electrode and the counter electrode, a negative second DC voltage component xcex94V2 is generated in the liquid crystal layer. To cancel this out, the potential of the common electrode standing at the counter potential level is shifted in a negative potential direction so as to be set at the correction potential level. In this way, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, so that the display quality improves.
Moreover, according to the invention, in a case where a transparent electrode is used as the pixel electrode, a negative second DC voltage component xcex94V2 is generated. Thus, the potential of the common electrode standing at the counter potential level is shifted in a negative potential direction so as to be set at the correction potential level. In this way, the DC voltage component acting upon the liquid crystal layer is kept as small as possible, so that the display quality improves.